In some connection-oriented protocols such as a TCP (transmission control protocol), an acknowledgement response (hereinafter, Ack) is returned in response to transmitted data. The time from when data is transmitted until the Ack is returned is referred to, for example, in a TCP, as “RTT (round trip time)”. When the RTT increases, the throughput decreases and a delay occurs. The delay is presumed to be caused by, for example, the physical distance between a transmitting apparatus and a receiving apparatus, the number of via-nodes between the transmitting apparatus and receiving apparatuses, and buffering during congestion and so on.
FIGS. 13A and 13B illustrate a delay due to buffering. In the example illustrated in FIGS. 13A and 13B, communication is performed in a single TCP session between a transmitting apparatus and a receiving apparatus.
In FIG. 13A, the window size of the transmitting apparatus is assumed to be 128 kbits. The RTT is assumed to be 0.1 second on average. In FIG. 13A, an average throughput (an average output rate) of the transmitting apparatus is derived by:Average Throughput=128 kbit/0.1 s=1.28 Mbps.
FIG. 13B illustrates an example in which a relay apparatus having a shaper with a shaping rate of 1 Mbps is provided between the transmitting apparatus and the receiving apparatus illustrated in FIG. 13A. In the example illustrated in FIG. 13B, at the relay apparatus, the input rate is 1.28 Mbps, whereas the output rate is limited to a shaping rate of 1 Mbps. Thus, a queue is formed in the buffer in the relay apparatus. As a result, a buffering delay, which does not occur in the example (illustrated in FIG. 13A) in which no shaper is provided, occurs in, for example, the following manner.
(1) For example, the packet length is assumed to be 1518 bytes. When two packets are evenly accumulated in the buffer in the relay apparatus, a delay due to buffering increases by about 0.024 s (1518×8×2 bit/1 Mbit).
(2) Since a delay at the relay apparatus increases by about 0.024 s, the RTT also increases by about 0.024 s to become 0.124 s (0.1 s+0.024 s).
(3) Since an increase in the RTT in the transmitting apparatus refers to an increase in the amount of time until the Ack is returned, the period of time from when the transmitting apparatus transmits one packet and receives the Ack with respect to the packet until the transmitting apparatus transmits a next packet increases. That is, the increases in the RTT reduce the average throughput of the transmitting apparatus. In the example illustrated in FIG. 13B, the average throughput of the transmitting apparatus decreases to about 1 Mbps, as is derived by:Average Throughput=128 kbit/0.124 s≈1 Mbps.
As a result of a reduction in the throughput of the transmitting apparatus to about 1 Mbps, the input rate of the relay apparatus in the example illustrated in FIG. 13B also decreases to about 1 Mbps. In the relay apparatus, the input rate decreases to the output rate, so that the input rate becomes substantially equal to the output rate.
As in the example illustrated in FIG. 13B, in a network in which window flow control is performed as in TCP, even when the input rate of the relay apparatus is higher than the output rate thereof, the input rate may decrease as a result of an increase in the amount of delay due to formation of a queue. A state in which the input rate of the relay apparatus and the output rate thereof become substantially equal to each other, as in the example illustrated in FIG. 13B, is referred to herein as an “equilibrium state”.
The equilibrium state is a phenomenon that can occur in any connection-oriented protocol employing an acknowledge response and is not a phenomenon limited to TCP window flow control as in FIGS. 13A and 13B.
Examples of related art include Japanese Unexamined Patent Application Publication Nos. 9-247213 and 2008-78932.